This invention relates to packaging of optoelectronic components which generate or process signals that pass through optical fibers. In particular, it addresses the critical need for providing stable, low-cost alignment of multiple optical fibers to a single packaged optoelectronic integrated circuit device, such as an optical amplifier.
An optoelectronic package is a container or housing that provides protection and support for both active and passive components contained within it. These components and their interconnection define an optical-electrical circuit and the function of the package. The package also includes a means of connecting the internal components with the external environment, usually as electrical feed-through and optical fiber. This invention is directed to the optical fiber and its connection to the components within the package.
To make an optical connection between an optical fiber and a component within a package, it is necessary to position or align the optical fiber in a way that allows efficient coupling between the optical fiber and the component. The precision needed for the alignment depends on the size of the light-emitting or light-receiving elements, the type of optical fiber, and any type of focussing or defocusing element which may be present, such as a lens on the optical fiber. An optical fiber transmits light through its inner core, which is much smaller than the diameter of the optical fiber. There are two classes of optical fiber presently used in packaging semiconductor devices: single-mode and multimode, with core diameters of 10 .mu.m and 100 .mu.m, respectively. Most telecommunication systems use singlemode fiber because it is superior in reducing noise arising from mode partition noise. Connecting an optical fiber to a semiconductor device is difficult. Extremely tight tolerances, on the order of 1 .mu.m, are required due to the small size (about one micron) of the active region of the semiconductor device. Additional problems arise when more than one optical fiber needs to be coupled to a single device, since this necessarily entails either simultaneous alignment or sequential alignment of the semiconductor device to multiple optical fibers. Simultaneous alignment is difficult because it requires a sudden "freezing" of the position of all optical fibers at once. Sequential alignment is costly and time-consuming, as will be explained infra.
Examples of semiconductor devices which require the coupling of multiple optical fibers are shown in FIGS. 1a and 1b FIG. 1a shows a top view of an array 10 of three semiconductor lasers, used as light sources for such purposes as parallel processing. The active layers are designated by the reference number 12 and the output beams by 14. An optical fiber must be coupled to each lasing output beam 14. FIG. 1b shows a top view of a tilted facet laser optical amplifier 15, having an active layer 16, which receives light at one end and outputs the amplified light at the other end. One optical fiber 17 must couple the light signal into the amplifier 15, and another optical fiber 18 must couple the amplified output. The amplifier 15 shown in FIG. 1b has facets 19 (mirrors) which are tilted 7 degrees with respect to the active layer 16. This tilt reduces the internal reflections in the active layer 16, and permits greater amplification of the light signal. (See C. E. Zah, C. Caneau, F. K. Sohkoohi, S. G. Menocal, F. Favire, L. A. Reith, and T. P. Lee, "1.3 .mu.m GalnAsP Near-Travelling-Wave Laser Amplifiers Made by Combination of Angled Facets and Antireflection Coatings", Electronics Letters 24, 1275 (1988), and J. LaCourse, W. Rideout, P. Gaslioli, and E. Meland, "1.3 .mu.m Tilted-Cavity Semiconductor Laser Amplifier", to be presented at SPIE OE/FIBERS '89, Boston, Mass. Sept. 5-8, 1989). However, the tilted facets 19 introduce another problem: for optimum coupling, the input and output optical fibers 17, 18 must be tilted with respect to the facet 19 by 23 degrees. This arises from Snell's law of refraction: EQU n.sub.air sin .theta..sub.air =n.sub.device sin .theta..sub.device
where n.sub.air is the refractive index of air,(approximately 1); .theta..sub.air is the peak angle of the beam in air (measured relative to the perpendicular to the facet); n.sub.device is the refractive index of the semiconductor material, typically approximately 3.3; and .theta..sub.device is the angle of the active layer inside the device (measured relative to the perpendicular to the facet). For example, if .theta..sub.device =7.degree. as shown in FIG. 1b, this equation mandates that the output beam 18 is tilted at .theta..sub.air =23.degree..
FIG. 2 shows coupling performance between a singlemode fiber and a high speed laser, a typical telecommunications component. Here, the position sensitivity of the optical fiber can be as little as 1 .mu.m, a size much smaller than the parts themselves. The prior art shows methods for such a precise alignment, but, until recently, for only one singlemode optical fiber per package. (See S. Enochs, "A Packaging Technique to Achieve Stable Single-Mode Fiber Laser Alignment", Proc. SPIE 703, 42 (1987) Other alignment methods make use of GRIN lenses, adding the complexity of aligning an additional optical element. (See L. A. Reith, J. W. Mann, and P. W. Shumate, "Design of a Low-cost Laser Package for Local Loop Applications Using Graded-Index Lenses", Proc. SPIE, 836, 327 (1988).) These prior art methods are costly, because of generally low yields and the fact that the alignment step is usually the last step in the manufacture of the package. The problem is compounded when more than one singlemode optical fiber must be aligned to the same package.
Prior art for multi-fiber alignment to a single package is predominantly concerned with the easier task of coupling large core multimode optical fiber to relatively large light sources and detectors. (See K. P. Jackson, A. J. Moll, E. B. Flint, and M. F. Cina, "Optical Fiber Coupling Approaches for Multi-Channel Laser and Detector Arrays", Proc. SPIE, 994, 40 (1988).) These alignments are less sensitive to position and can often be done with grooved parts and epoxy to fasten the optical fiber. This technology is acceptable for short length optical fiber links in local area networks or computers, but not for telecommunications.
Prior art also shows how laser welding can be used for connecting both singlemode and multi-mode optical fiber to packages. (See D. S. Bargar, "Automated Fiber Alignment, Fixing, and Hermetic Sealing System", Proc. SPIE, 994, 11 (1988).) The most recent art teaches two techniques for coupling multiple singlemode fibers to laser amplifiers. K. H. Cameron, et al, "Packaged Laser Amplifiers at 1.5 .mu.m for Submarine Systems", British Telcom Research Laboratories, Martiesham, Ipswich, UK (1989), disclose the use of sequential laser welding. Disadvantageously, this welding method calls for the investment in an expensive laser and is not suitable for reworking optical fibers which were originally connected misaligned, presenting a yield problem for packages with many optical fiber connections.
A second alignment technique for a multi-fiber laser amplifier package is taught by L. A. Reith, et al, "Single mode fiber coupling to a traveling wave laser amplifier", Bellcore, Morristown, N.J. (1989). In this technique, two GRIN lenses are used at both the input and the output. The addition of extra optical elements introduces complexity, alignment problems and additional expense.
Most recently, another approach requiring the manipulation of the component with respect to previously fixed optical fibers was published. (See K. Yoshino and M. Ikeda, "Novel Assembly Method for Laser-Diode Optical Switch Module", Electron. Lett. 25, 62 (1898).) This approach suffers because it requires a simultaneous alignment to two fixed optical fibers, requiring positional optimization of the semiconductor to a "compromise" position which may not be individually optimized for either optical fiber.
This specification discloses a new technique for sequential alignment, that is, aligning and "freezing" first one optical fiber, than aligning and "freezing" the second one, and so forth.